In most cases, high-performance optical elements which are extremely small are required in optical circuits and the like. In recent years, techniques in which photonic crystals are used in such microscopic optical elements have been widely researched. It is known that photonic crystals have extreme properties such as the complete reflection of light regardless of the direction of incidence due to a photonic band gap, variation in the velocity and direction of light due to abnormal dispersion, and that such crystals have numerous properties that are convenient for controlling light. Such photonic crystals and applications of photonic crystals are described in “Fotonikku Kessho Kenkyu no Genjo to Shorai Tembo Current Conditions and Future Development of Photonic Crystal Research” Optoelectronic Industry and Technology Development Association, March, 2000.”
Furthermore, Japanese Patent Application Kokai No. 2001-91701 discloses a technique in which an interference filter that transmits light of various wavelength regions is manufactured in a single substrate, or in which a waveguide type superprism with the center wavelength of the split light differing according to the incident position of the light is created, by varying one or more directions of the fundamental period of a periodic structural body either gradually or in gentle stages in spatial terms, or by varying one or more lengths of the fundamental period of a periodic structural body either gradually or in gentle stages in spatial terms in a photonic crystal, and a technique in which a bent waveguide path is formed by a similar method in a three-dimensional photonic crystal.
Although photonic crystals show conspicuous properties with respect to light, such functions are manifested only at interfaces between the photonic crystals and other media (as in the case of optical elements based on ordinary glass or crystals). In the interiors of the photonic crystals, light merely advances in a straight line at a fixed velocity. Accordingly, photonic crystals are not always suitable for the precise control of light. For example, in cases where a spectroscopic element is constructed, the wavelength range in which an abnormal dispersion is shown at a constant angle of incidence is limited, and in most cases, the dispersion is small elsewhere even compared to ordinary crystals. Accordingly, in ordinary photonic crystals, the splitting of light is limited to splitting the light in an extremely narrow band.
Furthermore, photonic crystals tend to show a loss caused by reflection, scattering, and the like at the boundaries with ordinary substances and other photonic crystals. This loss tends to be especially large in cases where abnormal dispersion is generated, and it is difficult to avoid such problems. Accordingly, in cases where optical systems are constructed using photonic crystals, it is important to heighten the function of one photonic crystal while minimizing the surfaces of the photonic crystals (i.e., reducing the number of photonic crystals).
Furthermore, the technique described in the above-mentioned Japanese Patent Application Kokai No. 2001-91701 involves the manufacture of an interference filter that allows the passage of light of various wavelength regions in a single substrate, and this patent application also discloses a technique for a waveguide type superprism in which the center wavelength of the split light differs according to the position of incidence. However, there is no mention of varying the refraction angle of the light according to the wavelength, or controlling the direction of advance of the light in the interior of the photonic crystal, by varying the structure of the crystal.